Something that is harmful causes or is capable of causing harm (no surprises there!). Something that is pernicious causes insidious harm or even ruin. This sneaky adjective describes things that are both highly destructive and not easy to detect—often because they develop or spread gradually or under the radar. The influence of a person or entity might be described as pernicious if it puts people or things in peril. Similarly, a pernicious myth is one that impairs understanding to the point of causing harm.
An inclination is a liking or preference for something. The synonym penchant is more pronounced and enduring: a penchant is a strong inclination, taste, or liking for something. Penchant is more likely to be used in reference to broad preferences that shape behavior over time. For instance, if a person has a penchant for secrecy, it means they tend toward secrecy not just in an isolated circumstance, but in their conduct more generally. Inclination, on the other hand, may be used to refer to a circumstantial preference: the employee’s inclination was to stay working on the project he started, rather than switch to something new.
The noun fettle is a somewhat old-fashioned way to refer to a person’s physical state or frame of mind. It’s almost always used in the positive with the descriptor fine: a person who is in fine fettle is feeling good and maybe even looking good, too. If you go looking for it, you may spot fettle in other sunny combinations, such as good fettle, excellent fettle, high fettle, and better fettle. Contrast that with the noun condition, which is indeed used to refer to a person’s state of health, but, in those uses, is more likely to be found alongside descriptors such as critical or poor.